The present invention relates to a digital information transmission system of the type which operates in a differential pulse code modulation mode, and more specifically to such a system which imposes constraints on selected signal values so as to increase the quality of transmitted digital information.
Differential pulse code modulation encoders operate by subtracting from a sample of an input signal a prediction signal corresponding to a linear combination of preceding samples of the input signal to thus generate a difference signal. The difference signal is then quantized, encoded and transmitted.
The quantized signal is also used by a predictor circuit to provide a feedback prediction signal corresponding to a linear combination of reconstructed samples of the input.
The predictor signals may achieve unrealizable levels, e.g., they may become negative or exceed the maximum value that the system can generate. Any system will impose its own minimum and maximum values below and above which it will not operate, but this is done only after processing of the entire image. Unrealizable data may thus be used in forming the predictions which in turn results in prediction errors and poor reconstructed image quality.
A patent of interest for its teaching of a differential pulse code modulation system which increases the accuracy of its prediction signal by utilizing the present signal value and the reconstructed signal value to generate the prediction signal is U.S. Pat. No. 4,385,393 entitled "Adaptive Prediction Differential PCM-Type Transmission Apparatus And Process with Shaping of the Quantization Noise" by C. Chaure et al.
Another patent of interest is U.S. Pat. No. 4,519,085 entitled "Differential Data Coding and Decoding Process and System Restricting Propagation of Transmission Errors" by J. Y. Catros.
The system of that patent minimizes the propagation of transmission errors by coding a transmitted signal with a forecasted error code word comprised in part of a quantized forecast value. The code word is transmitted and decoded to provide a forecast error component and a forecast value. The forecast value is used to reconstruct the original encoded value and the error component is used to compensate out errors in the calculation of the next forecast value.
Another patent of interest is U.S. Pat. No. 4,520,491 entitled "Transmission System Using Differential Pulse Code Modulation with Adaptive Prediction" by J. M. Raulin et al. In that patent quantized noise is reduced by providing an adaptive predictor whose response is adjusted to be the image of the spectrum of the signal to be transmitted which minimizes the accumulation of quantizing noise.